2001:1101 - GLASSHOUSE, Offaly

During this second season of excavation of the 17th-century glasshouse it was hoped to establish the limits of the glasshouse itself. Situated on an east-facing slope, the upstanding furnace would have been enclosed by a building, possibly wooden, though the wall foundation uncovered in the first season (Excavations 1999, No. 747) indicates that the southern side at least had a stone foundation. In order to follow the line of this wall two cuttings, 2m by 4m, were opened east and west of the line of the wall. In the eastern cutting the wall extended 1m into the cutting before terminating. There was no post-hole at the terminal to suggest that this had been an entrance and the wall trench did not reappear in the remaining 3m of the cutting. In fact the stratigraphy in the rest of this cutting was very shallow, with a thin layer of mortar appearing between the sod and the natural subsoil. The wall trench continued upslope through the western cutting, where it remained a consistent 0.9m wide and 0.15m deep. Further cuttings north and east of the upstanding furnace were opened in the hope of intercepting the limits of the wall trench in these areas.

In the northern cutting, 2m by 6m, an irregular linear feature running east–west was uncovered. A cutting to the east, 2m by 3m, and one to the west, 2m by 4m, were opened to follow this feature. However, it terminated in both cuttings. Though it is 3m from the area of intensive burning at the northern mouth of the furnace, it has been suggested that this was the foundation (0.2m deep) for some form of fire-screen wall (3.7m long), possibly wooden. This linear feature was cut into the natural sandy subsoil which appears to have been the internal floor of the glasshouse. Two trenches, 1m wide, were extended from these cuttings, 11m northward and 8m eastward, but no wall foundation was intercepted. However, the stratigraphy in these trenches was very shallow, with a layer of stones and debris beneath the sod and a layer of mortar beneath. Most of the mortar has a flat surface on at least one side, and in some cases on both sides. As there was no trace of a collapsed wall, it seems likely that this may have formed an area of hard internal flooring. In the northern extension, this mortar layer terminates 9.5m from the upstanding furnace. Immediately north of this termination point there appears to be a contemporary cobblestone surface. This mortar layer could also be followed in the eastern extension, beneath which is natural subsoil.

Another trench, 2m by 4m, was opened 14m north-east of the furnace. There are several undulations in the area surrounding the glasshouse and there was speculation that these may be mounds of glass debris or cullet, waste glass deliberately brought to glass-making sites to speed up the melting process. However, this cutting proved to be archaeologically sterile.

Unlike the downslope portion of the site east and north of the furnace, where the stratigraphy was very shallow, upslope the stratigraphy was over 1m deep. It would appear that the wall foundation, mentioned above, represents the last phase of activity on the site and is contemporary with the upstanding furnace. The wall foundation was cut into an earlier build-up of fire-reddened clay, possibly redeposited, and also partially ran along the edge of a pit full of debris, consisting of crucible, brick and furnace fragments, probably from an earlier furnace. This pit was sealed by a layer of mortar. Beneath the fire-reddened clay were thin layers of black and red charcoal-rich sandy soil containing window glass sherds. These may have been earlier working surfaces.

South-west of the furnace a cutting, 1m by 8m, was extended south of the wall foundation. Owing to time constraints this was not fully investigated; however, at a level contemporary with the wall foundation in the adjacent cutting, a stone feature, running east–west, parallel with the wall, was uncovered. A further cutting had been opened further upslope to the west to ascertain how far the site extended in this direction. Again this cutting was not fully resolved but a lot of scattered stones were uncovered 0.25m beneath the sod and seem to be suggestive of a collapsed wall. It would appear that the site extends at least 16m west of the upstanding furnace.

Finds were similar to those found in the first season (ibid.) and included window and vessel glass, though window glass was predominant. Also found were fragments of slates, some with nail-holes, which suggests that the glasshouse may have had a slate roof.

The natural sand which underlies the site has been analysed since the first season and was found to be a Carboniferous sand, not suitable for glass-making. It would appear that the sand needed for the glass manufacture had to be imported to the site.

This research excavation was grant-assisted by funding from Dúchas The Heritage Service, Department of Arts, Heritage, Gaeltacht and the Islands, on the recommendation of the National Committee for Archaeology of the Royal Irish Academy.

‘Sonas’, Curraghscarteen, Moyglass, Co. Tipperary and ‘Illaun View’, Glenbower, Coolbaun, Nenagh, Co. Tipperary.